1. Field of the Invention
This invention relates to a pneumatic radial tire comprising a carcass of a radial structure and a belt superimposed about a crown of the carcass and embedded in a tread rubber. In particular this invention relates to an improvement in the pneumatic radial tire using metallic cords, particularly steel cords as a main reinforcing element for the belt, which advantageously and largely enhances the service life of the tire by improving the buckling fatigue resistance and corrosion resistance of the metallic cord.
2. Description of the Prior Art
The metallic cord of this type closest to the invention, are steel cords each comprising a core of two equal diameter metallic wires and an outer layer of six equal diameter metallic wires disposed about the core, the diameter of the outer layer wires being generally about 1.23.about.1.43 times the diameter of the core wires. Such are described in U.S. Pat. No. 3,996,733.
The inventors have made studies with respect to such steel cords used in a belt layer of a radial tire and confirmed that the buckling fatigue resistance and corrosion resistance are not sufficiently ensured in this steel cord. At present, it is strongly demanded to further improve steel cords.
Also, there have hitherto been known steel cords for use in the radial tire each comprising a core of two equal diameter metallic wires twisted with each other, an outer layer of seven equal diameter metallic wires twisted about the core in the same twisting direction as in the core wires and a spiral wrap of a single metallic wire twisted therearound. However, this steel cord still has insufficient buckling fatigue resistance and corrosion resistance likewise the aforementioned case.
The serious drawback of the steel cord according to the aforementioned U.S. Pat. No. 3,996,733 results from such a different diameter structure where the diameter of the outer layer wire is about 1.23.about.1.43 times the diameter of the core wire.
Concerning buckling fatigue resistance as a characteristic particularly required in the belt of the radial tire, it has been found that the buckling fatigue resistance and corrosion resistance considerably lower as the ratio in the diameter of the outer layer wire to the core wire becomes larger. This results from the conspicuous buckling repeatedly produced in the cornering and the like during the running of the radial tire. When buckling is produced in the steel cords as a reinforcing element for the belt, the maximum strain of the buckling is naturally given to the metallic wire in the outermost layer of the steel cord. Consequently, as the diameter of the metallic wire in the outermost layer becomes large, the maximum strain increases to reduce the life of the buckling fatigue resistance.
Further, as the ratio in diameter of the outer layer wire to the core wire increases, the space between the metallic wires constituting the outer layer reduces and consequently the penetrability of rubber constituting a belt layer to the inside of the steel cord lowers considerably. As a result, the rubber coating ratio on the core of the steel cord is also reduced, so that there is considerably increased the corrosion of the steel cord due to penetrated water during the running of the radial tire for a long time or water penetrating through tread cuts produced by treading on sharp rocks or through a nail penetrated into the tread.